Apparatus and system for spacecraft celestial navigation using spectral observations of extrasolar planetary systems

ABSTRACT

The present invention provides an innovative apparatus and system for onboard spacecraft location determination and celestial navigation by employing spectral observations of extrasolar planetary star system motion. In one apparatus embodiment a gas absorption cell is placed between a sensor and the light from a reference star system with at least one exoplanet, such that the sensor can detect the spectrum through the gas absorption cell. Radial velocities can be calculated via Doppler Spectroscopy techniques and incorporated into a spacecraft navigation solution. The present invention can enable and enhance significant mission capabilities for future manned and unmanned space vehicles and missions.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims a benefit of priorityunder 35 U.S.C. §120 of the filing date of U.S. patent application Ser.No. 13/538,655 filed on Jun. 29, 2012, now abandoned which in turnclaims the benefit of priority under 35 U.S.C. §119 to U.S. ProvisionalPatent Application No. 61/571,554 filed Jun. 30, 2011, the entirecontents of which are hereby expressly incorporated by reference for allpurposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is an innovative apparatus, system and method forspacecraft navigation employing the use of extrasolar planetary systemmotion. Spacecraft navigation can generally be described as, but notlimited to, the determination of a spacecraft's position, velocity andattitude at certain times as well as the determination of orbitalparameters and trajectories. Extrasolar planetary systems are starsystems other than the Sun that have planetary companions. The presentinvention relates to several different fields including spacecrafthardware, software, navigation, astronomy, Doppler spectroscopy methodsand astrometric techniques.

2. Description of the Related Art

Precise determination of spacecraft position and velocity is necessaryin order to achieve mission success for operations of near Earth andinterplanetary missions. Onboard flight technologies can providespacecraft position, navigation and timing (PNT). Areas of related artinclude traditional spacecraft navigation hardware and software,tracking such as NASA's Deep Space Network (DSN), the Global PositioningSystem (GPS), X-ray navigation and extrasolar planetary detection.

Space navigation traditionally relies on initial spacecraft position,velocity and attitude estimates that are regularly updated by onboardinertial measurement unit (IMU) data. An IMU is a device that measures aspacecraft's velocity changes and orientation using a combination ofaccelerometers and gyroscopes. Spacecraft orientation can also be aidedby a star tracker, which is an optical device that measures the relativeposition(s) of star(s) against the celestial background using photocellsor a charged couple device (CCD) camera. Additional components such ashorizon or sun sensors are also traditionally employed.

Methods of onboard orbit and position determination involve accurateupdates to the spacecraft's navigation state matrix (“Nay State”).Periodic updates from external signals can be processed by onboardsoftware algorithms and filters. As an example, in low Earth orbit(LEO), the Nay State can be refined by employing Kalman filtering anddata from terrestrial navigation aids such as C band radar tracking orthe GPS. There are various ways to implement these software filteringcapabilities, one of which is NASA's GPS Enhanced Onboard NavigationSoftware (GEONS).

GEONS supports the acceptance of many one way forward Doppler, opticalsensor observation and accelerometer data types. GEONS was designed forautonomous operation within the limited resources of an onboardcomputer. It employs an extended Kalman filter (EKF) augmented withphysically representative models for gravity, atmospheric drag, solarradiation pressure, clock bias and drift to provide accurate stateestimation and a realistic state error covariance. GEONS incorporatesthe information from all past measurements, carefully balanced with itsknowledge of the physical models governing these measurements, toproduce an optimal estimate of a spacecraft's orbit. GEONS'high-fidelity state dynamics model reduces sensitivity to measurementerrors and provides high-accuracy velocity estimates, permittingaccurate state prediction.

Interplanetary missions typically employ tracking services from NASA'sDSN, which provides radiometric ranging, Doppler and plane-of-sky anglemeasurements. For spacecraft ranging, a signal is sent from one of theDSN stations on Earth to the spacecraft, which in turn sends a signalback to Earth. The round trip transit time is measured to determine theline of sight slant range. Two-way Doppler tracking also uses a signalsent to and from a spacecraft; by looking at the small changes infrequency, the spacecraft velocity along the line of sight can bedetermined.

In general, angular measurements can be made using multiple DSN groundstations that receive spacecraft transmissions simultaneously duringoverlapping viewing periods. An additional method used by DSN is deltadifferential one-way range (ADOR). This is a Very Large BaselineInterferometry (VLBI) technique that uses two ground stations tosimultaneously view a spacecraft and then a known radio source (such asa quasar) to provide an angular position determination.

Unfortunately, DSN resources are limited and its accuracies degrade overlarge distances. Onboard spacecraft navigation systems that can reducetracking requirements for the DSN are currently needed. Furthermore, GPSsatellites orbiting the Earth are of limited use for deep spacemissions. Thus, hardware and software systems and methods that provideprecise navigation solutions using a methodology that is independent ofEarth based systems are not only innovative and novel but are currentlyneeded for spacecraft navigation.

Some recent research and development with autonomous deep spacenavigation has examined the use of pulsed X-ray radiation emitted bypulsars. Such investigations designate X-ray millisecond pulsars as apotential signal source to be observed by a spacecraft. However, thespecific characteristics of pulsars are limiting and very different frommain sequence stars such as our sun. The current invention uses theproperties of main sequence stars and their associated extrasolarplanets.

In the past 15 years or so, over 700 extrasolar planets (or exoplanets)have been discovered orbiting around 560 main sequence stars (some starshave multiple detected exoplanets). These stars are evenly distributedthroughout the celestial sphere and most are within several hundredlight years (ly) of Earth. Some potential exoplanet reference starsinclude, but are not limited to, Epsilon Eridani (10 ly away), Gliese 86(36 ly), 47 Ursae Majoris (43 ly), 55 Cancri (44 ly), Upsilon Andromedae(44 ly), 51 Pegasi (48 ly) and Tau Bootis (49 ly). All have well knowncharacteristics and are even visible to the naked eye.

Before the discovery of exoplanets, the only planets known to exist werethose in our own solar system. The motion of the Earth about our Sun iswell understood and the whole solar system in fact rotates around acommon center of mass, known as the barycenter. Astronomers, in order todetect possible planets around stars other than our Sun, had to separateknown and unknown stellar motion to determine the motion of other starsabout their own barycenters. The initial theory postulated that ifexoplanets did exist, their orbits would cause their parent star towobble by a small amount. This motion was indeed detected, yieldingnumerous exoplanet discoveries. The measurements to date have producednow well known patterns of highly stable, predictable exoplanetarysystem stellar motion with respect to our own solar barycenter. Thisexoplanetary system stellar motion can be used to determine the locationof a spacecraft both within and outside of our solar system. This is themethodology employed by the present invention.

SUMMARY OF THE INVENTION

The present invention is an apparatus, system and method for spacecraftlocation determination and navigation employing extrasolar planetarysystem motion. The apparatus, system and method provide onboard orbit orlocation determination and navigation capabilities during spacecraftoperations through the use of specialized reference stars that haveexoplanet companions. The motion of these exoplanets around thereference star's barycenter provides a stable, highly predictablenatural signal pattern. The measurements of these signal patterns aretaken onboard the spacecraft and are used with onboard softwarealgorithm estimation techniques to determine both spacecraft locationand navigation. The present invention enables and enhances significantmission capabilities for future manned and unmanned space vehicles aswell as reducing DSN tracking requirements and resources.

The present invention can provide primary or secondary navigationcapabilities for space missions. It is expected to provide positionalsolutions anywhere within the solar system as well as beyond our solarsystem. Primary autonomous navigation can be incorporated intospacecraft designed for geostationary, elliptical high earth orbits, ordeep space orbits or trajectories. Back-up or secondary navigationcapabilities could be available for emergency situations in low andmedium Earth orbits when primary navigation is lost (such as in the caseof denied access to GPS). The present invention could be used for mannedmissions and would be particularly useful at locations currently ofinterest such as lunar orbits, asteroids, comets, libration points,Martian moons or outer solar system planets.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings, in which:

FIG. 1 illustrates Solar motion about the barycenter, from the timeperiod of 1960 to 2025 AD.

FIG. 2 illustrates the radial velocity of the Sun as it orbits the solarsystem barycenter.

FIG. 3 illustrates a spacecraft in the space environment.

FIG. 4 illustrates a functional spacecraft block diagram.

FIG. 5 illustrates the components of a standard star tracker.

FIG. 6 illustrates an exoplanetary star tracker apparatus and gasabsorption cell block diagram.

FIG. 7 illustrates the principle elements of an astrometricinterferometer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Nay State determination through the use of extrasolar planetary systemmotion data is an innovative method for onboard spacecraft navigation.It will significantly enable and enhance mission capabilities for futuremanned and unmanned space vehicles as well as reducing the need for DeepSpace Navigation resources. Over 700 extrasolar planets have beendiscovered around nearby main sequence stars within the past 15 years.The motion of these extrasolar planets around their stellar barycentersprovides a stable, highly predictable natural signal pattern.Observations from these star systems allow for enhanced spacecraft selfdetermination of orbits and position as well as navigation.

Extrasolar Planetary System Motion and Measurements

Earth based exoplanet searches have sought to identify planetary systemsby observing characteristics of the parent star about which thepotential planet is orbiting. The main methodologies employed for suchexoplanet detection have been astrometry and Doppler spectroscopy. Incelestial mechanics, the simplest case is of a single planet orbitingaround one star. The system orbital parameters can be derived fromEquation 1:a ³=(M _(*) +m _(p))P ²  (1)where the masses (M_(*), m_(p)) are in solar units, the semi-major axis(a) is in astronomical units (AU) and the period (P) is in years. Themotion of the star is much smaller than that of the associated planet.Using techniques for indirect observation of exoplanets, the smallmotion of the reference star is detected, allowing for calculations thatinfer the existence of the exoplanet.

Astrometry attempts to measure the movement of a star with respect tobackground stars. In cases where the movement is apparent, parallax isbeing measured. If a star were seen to have an elliptical motion, theprobable explanation would be that the wobble is due to a star orbitingabout its barycenter. Using Equation 1 and the fact that the semi-majoraxis can be measured as an angle, θ, yields Equation 2:

$\begin{matrix}{\theta = {{\frac{m_{p}}{M_{*}}\frac{a}{r}} = {\frac{m_{p}}{r}\left( \frac{P}{M_{*}} \right)^{2/3}}}} & (2)\end{matrix}$where θ is in arcsec when a is in AU, both masses are in solar units,distance (r) is in parsecs (pc) and P is in years. For example, if onewere to view our solar system from a distance of 10 pc, Jupiter wouldappear as an 11.9 year disturbance in the Sun's motion with a 0.5milliarcsec amplitude. FIG. 1 displays what our solar system motionabout its barycenter would look like if viewed from the north eclipticpole at a distance of 10 pc, with the right horizontal axis pointing tothe Vernal Equinox. Planet detection is most sensitive to stars that arenear the solar neighborhood and have a large planet. Most of theexoplanets detected to date have been described as “large Jupiters”,with periods measured in days.

For astrometry, the motion of the star is most pronounced when theexoplanet(s) orbiting the star are in a plane perpendicular to the lineof sight of the observation point. Any other orientation would producesome cyclical motion towards and then away from the observation point.Doppler spectroscopy takes advantage of this radial motion by trying todetect the alternating red and blue spectrum shifts that a star in thisorientation would have. This Doppler motion would create a variableradial velocity as dictated by Equation 3:

$\begin{matrix}{v = {{30\frac{\;{m_{p}\sin\; i}}{\left( {aM}_{*} \right)^{1/2}}} = {30\frac{m_{p}\sin\; i}{M_{*}^{2/3}P^{1/3}}}}} & (3)\end{matrix}$where v is in km/sec, the masses are in solar units, a is in AU, P is inyears and i is the inclination of the orbit to the plane of the sky.Using the previous example for astrometry, Jupiter has a velocityvariation of 13.0 m/sec over a period of 11.9 years. Most exoplanetsdetected to date have larger velocity variations than Jupiter, over aperiod of just days. FIG. 2 depicts the apparent radial velocity shiftof our Sun, primarily due to Jupiter, as viewed from the Vernal Equinoxfor the same time period as shown in FIG. 1.

Doppler spectroscopy measurements are thus exceptionally useful, sinceidentified stars with planetary companions have a stable, knownrepeatable pattern of motion. Astrometric measurements of parallax andstellar angular displacements also provide valuable data. Since thesestellar motions about the barycenter are known with a high degree ofprecision and consistently and reliably repeat over many cycles andyears, they make excellent reference sources. Currently there are over500 observed exoplanet star systems. This population allows for a viableextrasolar planetary system reference database for onboard spacecraftnavigation.

Full three dimensional absolute and relative navigation solutions areachievable from extrasolar planetary system sources, including positionand velocity determination as well as spacecraft attitude determination.Spacecraft navigation algorithms and software filtering can combineonboard measurements with exoplanetary stellar motion based models andother characteristics, such as source declination, right ascension andproper motion to yield a solution. Absolute position or delta updates toa position can be calculated and blended with a spacecraft's Nay State.

Absolute positions may be obtained either by range or wavelength phasemeasurements. In general, a spacecraft range (p) can be calculated fromthe difference in the transmit and receive times of one source spectrumby Equation 4:ρ=c(t _(r) −t _(t))  (4)where c is the speed of light. If the range measurement is known as wellas the unit vector for the extrasolar planetary system source, thespacecraft range in an inertial reference system may be computed.Absolute position can also be achieved through simultaneous observationsof several sources. Determining the range measurements of any unique setof three extrasolar planetary systems yields the location of aspacecraft in three dimensional space.

Wavelength phase measurements can be thought of as a total wavelengthphase that is the sum of some integer number of cycles plus a fractionof one cycle. These measurements and their time of arrival can be mergedand used by navigation software to determine position by employing aprocess similar to GPS integer cycle ambiguity resolution. The basicequation for GPS carrier phase pseudorange is well known in theliterature and can be written as Equation 5:Φ=[1/λ]ρ+fΔδ+N  (5)where Φ is the measured carrier phase, N is the phase ambiguity integeror “integer ambiguity”, Δδ is the clock bias, λ and f are the GPScarrier phase wavelength and frequency, and ρ is the range. Substitutingf=c/λ and expressing Equation 5 as a mathematical model yields Equation6 and Equation 7:Φ_(ij)(t)=[1/λ]ρ_(ij)(t)+[c/λ]Δδ _(ij)(t)+N _(ij)  (6)where i and j are two points in a designated reference frame at an epoch(t) and:ρ_(ij)(t)=[(X _(j)(t)−X _(i))²+(Y _(j)(t)−Y _(i))²+(Z _(j)(t)−Z_(i))²]^(1/2)  (7)

While the above equations are usually applied to GPS and its geocentricreference frame, the same concepts are employed for the spaceenvironment for the purposes of this invention. The wavelength selectedcould be any one of many that are associated with the stellar signatureof an extrasolar planetary system and the coordinates can be in aninertial solar reference frame tied to the solar barycenter. Using thistype of solar reference frame and an appropriate timing model defined ata specific location, information observed at a spacecraft can be matchedwith data in an onboard extrasolar planetary system database to providea navigation solution.

Furthermore, onboard software algorithms may employ differencingtechniques for one or more extrasolar planetary systems to removeerrors. A single difference calculation could be done between themeasured spacecraft wavelength phase arrival and the phase predicted ata model location. A double difference could be obtained by subtractingtwo single differences from two different sources. A triple differencecould be calculated by subtracting two double differences from twoseparate time epochs.

It is also noted that the observed star radiates in the entireelectromagnetic spectrum, so multiple wavelengths can be monitored atthe same time. This would provide for naturally occurring multiplefrequencies from the source, similar to GPS satellites broadcasting morethan just one L band frequency.

Exoplanetary System Star Tracker Apparatus for Space Navigation

FIG. 3 depicts a partial representation of the space environment, withthe Earth 1 orbiting the Sun 2. A spacecraft 3 is also depicted, withthe disclosed inventions located onboard. An inertial solar referenceframe 4 is shown with the origin located at the solar system barycenter.The distances to the Earth, Sun and spacecraft in the reference frameare indicated by ρ_(E) ρ_(S) and ρ_(sc) respectively. Some extrasolarplanetary systems 5 are viewable from the spacecraft. Each independentextrasolar planetary system 5 would have a known unit vector in theinertial reference frame as well as a known stellar signature.

FIG. 4 depicts a spacecraft functional block diagram of one embodimentof the invention. A spectrum wavelength λ_(eps) from one or moreextrasolar planetary system sources 6 is viewable from the spacecraft 3.The spacecraft has an onboard computer 7 with hardware components suchas, but not limited to, processor(s), memory, storage, busses, powersources, oscillators and/or timing sources. The onboard computer 7 alsohas software processing capabilities and algorithms that perform variousnavigation functions such as, but not limited to, signal processing,clock adjustments, ephemeris and model propagation and filteringcorrections (such as least squares or Kalman) to improve position andvelocity estimates.

The spacecraft 3 also has other subsystems 8. Subsystems 8 may include,but are not limited to, navigation units such as IMUs, star trackers,GPS receivers, horizon and sun sensors. Subsystems 8 may also include,but are not limited to, scientific instruments, guidance units,thrusters, propulsion engines and communication systems. A data bussystem 9 connects the onboard computer 7 to the spacecraft subsystems 8as well as to one or more extrasolar planetary system star trackers,depicted as 10, 11 and 12 in FIG. 4. If more than one extrasolarplanetary system star tracker is located on a spacecraft, theorientation of their axes and fields of view may be chosen to optimize afunction such as, but not limited to, viewing different sources orredundancy. An extrasolar planetary system star tracker or sensor may becomprised of various components such as, but not limited to, photocells,CCDs, gas absorption cells, processor(s), memory, storage, busses, powersources and oscillators.

The present invention incorporates advancements to traditional startrackers that have been used in the aerospace industry. These startrackers have been integrated into spacecraft platforms and mostapplications to date have used them for corrections to IMU or ring lasergyro derived spacecraft attitudes. Individual star trackers have alsobeen used during the approach phase of rendezvous operations to update aspacecraft's relative Nay State. FIG. 5 depicts a typical star tracker.Major components usually include a light shade 13, a bright objectsensor 14, a shutter mechanism 15, a protective window 16, an adapterplate 17, and a main assembly instrument section 18 with connectors 19.

The present extrasolar planetary system star tracker invention couldstill be employed for traditional uses. However, the greatest benefitsare derived from the innovative approaches implemented in the instrumentpackage, namely orbit and location determination and navigationcapabilities through utilization of Doppler spectroscopy and/orastrometry. Doppler spectroscopy is achieved by placing a gas absorptioncell or other similar device in the star tracker field of view. Anotherembodiment would allow potential astrometric data to be obtained with aphoton collector or a Michelson interferometer. A navigation solution isdetermined or refined by the radial velocities produced by Dopplerspectroscopy of a reference star with exoplanets and/or astrometricangular displacements and parallax measurements.

An embodiment of the present invention may use single aperture and/orinterferometric equipment for astrometric measurements. Radial velocitydetection for Doppler spectroscopy may use the Fabry-Perot and/or gasabsorption cell techniques. The preferred embodiment of the presentinvention star tracker system would make use of an I₂ gas absorptioncell. The I₂ gas absorption cell technique has been successful in theEarth based detection of exoplanets. The main components consist of atranslucent glass cell, heaters, temperature sensors, insulation andnecessary electronics.

FIG. 6 depicts a block diagram preferred embodiment of an extrasolarplanetary system star tracker with a gas absorption cell apparatus.Iodine gas is enclosed in a central tube 20 and the whole cell housing21 is placed in the path of the stellar spectrum 6 being observed. Thespectrometer CCD 22 records the photons detected in the designatedwavelengths for both the stellar spectrum 6 and the I₂ gas cell spectrum23. The electronic package 24 may be comprised of various componentssuch as, but not limited to, processor(s), memory, storage, busses,power sources, oscillators as well as software algorithms and programs.The pure stellar spectrum template is eventually compared to thecombined I₂ gas cell and stellar spectrum to derive the necessary radialvelocities

With the present invention, data could also be collected from apotential astrometric interferometer. Most existing star trackers areset up to detect some minimum light flux intensity and then record thelocation of the light in the star tracker's field of view.Interferometers obtain data in another manner. The present inventionapparatus may have various embodiments with an interferometer, eitherwithin the extrasolar planetary system star tracker apparatus itself,several devices located on the spacecraft platform or devices located onmultiple spacecraft.

Referring to FIG. 7, light from the target star is collected by twosubapertures and routed via mirrors to a beam splitter (a partiallyreflective mirror) where the two beams are combined. This combined beamwill exhibit constructive and destructive interference; the interferencewill be at a maximum if there are equal optical path lengths from thesource to the beam splitter via the two arms. If the source direction isshifted relative to the interferometer baseline, an additional pathdelay results in one beam external to the interferometer. This pathdelay must be compensated by an equal amount of path delay in the otherbeam internal to the interferometer to maintain the maximuminterference. This relationship can be written as Equation 8:X=B·S+C=|B|sin θ+C  (8)where B is the baseline vector (essentially the vector connecting thetwo subapertures), S is the unit vector to the star, C is a constant(instrument bias) and the delay X is the amount of internal path lengthnecessary to equalize the path delays. Thus, the delay X is a measure ofthe angle between the interferometer baseline and the star unit vector.

The present invention apparatuses, systems and methods disclosed in thisapplication are envisioned to have multiple forms, steps andembodiments. These can include, but are not limited to, variousmodifications, separate and/or integrated components, chipsets, boards,sensors and computer architectures as well as similar or analogoushardware and software.

The invention claimed is:
 1. An extrasolar planetary star trackerapparatus for a spacecraft which allows the spectral observation of anextrasolar planetary star system with at least one exoplanet comprising:a housing for a gas absorption cell placed in the path of the extrasolarplanetary star system spectrum being observed; a gas absorption cellplaced in the housing configured to receive the spectrum from theextrasolar planetary star system with at least one exoplanet; aspectrometer used to detect the extrasolar planetary star systemspectrum from the gas absorption cell; and a data bus connecting thespectrometer to an electronics assembly, wherein the electronicsassembly is comprised of a processor and memory used to compare thespectrum detected by the spectrometer to an onboard extrasolar planetarystar system reference database to derive an onboard spacecraftnavigation solution.
 2. The apparatus of claim 1, wherein the spectrumdetected by the spectrometer is used to calculate radial velocitiesusing Doppler spectroscopy.
 3. The apparatus of claim 1, wherein thespectrum detected by the spectrometer is used to calculate spacecraftvelocity.
 4. The apparatus of claim 1, wherein the spectrometer detectedspectrum measurements are used to calculate a filtered estimate ofspacecraft position.
 5. The apparatus of claim 1, wherein the extrasolarplanetary star system with at least one exoplanet is used to calculatespacecraft attitude.
 6. The apparatus of claim 1, wherein an additionaldata bus is connected to the electronics assembly to disseminatedetected measurements to spacecraft systems.
 7. A spacecraft celestialnavigation system using spectral observations of extrasolar planetarystar system motion comprising: an extrasolar planetary star trackerapparatus which allows the observation of a spectrum from an extrasolarplanetary star system with at least one exoplanet comprising: a housingfor a gas absorption cell placed in the path of the extrasolar planetarystar system spectrum being observed; a gas absorption cell placed in thehousing configured to receive the spectrum from the extrasolar planetarystar system with at least one exoplanet; a spectrometer used to detectthe extrasolar planetary star system spectrum from the gas absorptioncell; a data bus connecting the spectrometer to an electronics assembly;and an additional data bus that connects the extrasolar planetary startracker apparatus to a computer located onboard the spacecraft; whereinthe computer located onboard the spacecraft is comprised of a processorand memory, and the computer located onboard compares the extrasolarplanetary star system spectrum detected by the spectrometer to anonboard extrasolar planetary star system reference database forderivation of a spacecraft navigation solution.
 8. The system of claim7, wherein the computer located onboard the spacecraft is used tocalculate a filtered estimate of spacecraft position.
 9. The system ofclaim 7, wherein the computer located onboard the spacecraft calculatesspacecraft position using a Kalman filter.
 10. The system of claim 7,wherein the computer located onboard the spacecraft calculatesspacecraft position with additional navigation sensor measurements. 11.The system of claim 7, wherein the computer located onboard thespacecraft is used for controlling the velocity of the spacecraft. 12.The system of claim 7, wherein the extrasolar planetary star system withat least one exoplanet is used to calculate spacecraft attitude.
 13. Thesystem of claim 7, wherein there are one or more additional extrasolarplanetary star tracker apparatuses connected to the additional data busand the computer located onboard the spacecraft.
 14. The system of claim13, wherein each of the extrasolar planetary star tracker apparatusesare used to observe a different extrasolar planetary star systemsimultaneously.
 15. A spacecraft celestial navigation system utilizingthe spectral observations of extrasolar planetary star system motion toderive spacecraft navigation solutions comprising: one or moreextrasolar planetary star trackers, each containing a housing for a gasabsorption cell placed in the path of the extrasolar planetary starsystem spectrum being observed and a spectrometer used to detect theextrasolar planetary star system spectrum from the gas absorption cell;and a data bus connecting the one or more extrasolar planetary startrackers to a computer located onboard the spacecraft; wherein thecomputer located onboard the spacecraft compares the extrasolarplanetary star system spectra detected by the one or more spectrometersto an onboard extrasolar planetary star system reference database toderive a spacecraft navigation solution.
 16. The system of claim 15,wherein the computer located onboard the spacecraft calculatesspacecraft position using a Kalman filter.
 17. The system of claim 15,wherein the computer located onboard the spacecraft calculatesspacecraft position with additional spacecraft navigation sensormeasurements.
 18. The system of claim 15, wherein the computer locatedonboard the spacecraft is used for controlling the velocity of thespacecraft.
 19. The system of claim 15, wherein the extrasolar planetarystar system with at least one exoplanet is used for calculatingspacecraft attitude.